An assembly of at least 2 metallic substrates

ABSTRACT

A method for the manufacture of an assembly of at least two metallic substrates spot welded together through at least one spot welded joint, such method including two steps, the assembly obtainable according to this method and the use of this assembly for the manufacture of automotive vehicle.

The present invention relates to a method for the manufacture of anassembly of at least two metallic substrates and the assembly. Theinvention is particularly well suited for the manufacture of automotivevehicles.

BACKGROUND

With a view of saving the weight of vehicles, it is known to use highstrength steel sheets to achieve lighter weight vehicle bodies andimprove crash safety. Hardened parts are also used notably to reduce theweight of vehicles. Indeed, the tensile strength of these steels is of aminimum of 1200 MPa and can be up to 2500 MPa. Hardened parts can becoated with an aluminum-based or zinc-based coating having a goodcorrosion resistance and thermal properties.

Usually, the method for the manufacture of a coated hardened partcomprises the following steps:

-   -   A) the provision of a steel sheet pre-coated with a metallic        coating being conventional coating based on aluminum or zinc,    -   B) the cutting of the coated steel sheet to obtain a blank,    -   C) the thermal treatment of the blank at a high temperature to        obtain a fully austenitic microstructure in the steel,    -   D) the transfer of the blank into a press tool,    -   E) the hot-forming of the blank to obtain a part,    -   F) the cooling of the part obtained at step E) in order to        obtain a microstructure in steel being martensitic or        martensito-bainitic or made of at least 75% of equiaxed ferrite,        from 5 to 20% of martensite and bainite in amount less than or        equal to 10%.

It is generally followed by the welding of two coated hardened parts orone coated hardened part with another metallic substrate. The welding ofaluminum or zinc based coated hardened parts is very difficult torealize due to the coating being hard and thick.

The patent application EP3020499 discloses a resistance spot weldingmethod comprising:

-   -   a pulsation process of clamping a sheet assembly of two or more        overlapping steel sheets including at least one high-tensile        steel sheet using a pair of welding electrodes that are        connected to a spot welding power source employing an inverter        direct current method, and performing a plurality of repetitions        of current passing and current pass stopping, while pressing the        steel sheets with the welding electrodes; and    -   a continuous current passing process in which, after the        pulsation process, current is passed continuously for a longer        period of time than a maximum current passing time of the        pulsation process, while pressing the steel sheets with the        welding electrodes.

However, this method is only dedicated to hot stamped steel sheetscoated with conventional zinc-based-coatings and aluminum-basedcoatings. Indeed, in Examples, this method was tested on aluminum coated1500 MPa hot stamped steel sheets, galvannealed coated 1500 MPa gradehot stamped steel sheet and ZnO skin-treated Al coated 1500 MPa gradehot stamped steel sheet. Specific coatings based on aluminum or zincincluding other elements are not included in this patent application.

The patent application EP3085485 discloses a resistance spot weldingmethod welding a plurality of steel sheets including a high tensilesteel sheet superposed, in which said resistance spot welding method,the conduction system is pulsation conduction using an inverter DCwelding power supply, and, in the plurality of current pulses formingthe pulsation conduction, at the respective current pulses, theconduction time, the intervals of the current pulses defined as theconduction idle time, and the weld currents applied by the currentpulses are variably controlled.

However, this method is dedicated to hot stamped steel sheets comprisingon its surface a solid solution of intermetallic compounds and iron byan alloying reaction between a conventional zinc-based (pure Zn, Zn—Fe,Zn—Ni, Zn—Al, Zn—Mg, Zn—Mg—Al, etc.) or a conventional aluminum-based(Al—Si etc.) coating and the steel of the base material. These surfacesare formed with an oxide layer mainly comprised of zinc or aluminum.Further, sometimes the surface of the coating mainly comprised ofintermetallic compounds of iron and aluminum is formed with a filmmainly comprised of zinc oxide. In Examples, the method was tested onhot stamped steel sheets coated with an alloyed of aluminum coatingcomprising 9% by weight of Si and Fe and a very small amount of ZnO, andon galvannealed coated hot stamped steel sheets. Usually, the nativeoxide layer of these coatings has a thickness between 10 and 100 nm.When a thin layer of ZnO is deposited on the aluminum based coatedhardened part before the austenitization, ZnO and the aluminum-basedcoating are alloyed. Since a very thin layer of ZnO is deposited on thealuminum-based coating, the oxide native mainly composed of Aluminum isstill very thin after austenitization, i.e. 10-100 nm, leading to aneasy welding. Specific coatings based on aluminum or zinc containingother elements are not included in this patent application.

The patent application GB2468011 discloses a method for applying acurrent for resistive welding of a plate assembly in which a material ofat least one plate is a high-tensile material, the method comprising:

-   -   a first step of applying a first amperage of a magnitude that        softens a surface of a joining location of the high-tensile        material, continuously for a first predetermined duration;    -   a second step of, when the first predetermined duration has        passed, switching an energization amount from the first amperage        to a second amperage that causes a nugget to grow at the joining        location; and    -   a third step of applying the second amperage continuously for a        second predetermined duration.

This method is dedicated to a high-tensile material or a hot-stampedmaterial. The hot-stamped material can be coated with a plating layer.However, the nature of the plating layer is not specified. Additionally,in the first step, low amperage is applied to soften the surface of ajoining location and the second step, high amperage is applied to causethe growth of the nugget at the joining location of the high tensilematerials. Nevertheless, the low amperage in the first step does notallow the welding of specific coated hot stamped parts wherein thecoatings comprising other elements than zinc or aluminum.

Recently, new coatings have been developed for hot formed steel sheets.The patent application WO2017/017521 discloses a phosphatable hardenedpart coated with an alloyed coating comprising from 0.4 to 20.0% byweight of zinc, from 1.0 to 3.5% by weight of silicon, optionally from1.0 to 4.0 by weight of magnesium wherein the ratio Zn/Si is between 3.2and 8.0. The patent application WO2017/017514 discloses a hardened partcoated with an alloyed coating comprising from 2.0 to 24.0% by weight ofzinc, from 1.1 to 7.0% by weight of silicon and optionally from 1.1 to8.0% of magnesium, the balance being aluminum wherein the ratio Al/Zn isabove 2.9 for improving the liquid metal embrittlement (LME) resistance.The patent application W02017/017513 discloses a sacrificial steel sheetcoated with a coating comprising from 2.0 and 24% by weight of zinc,from 7.1 to 12.0% of silicon, optionally from 1.1 to 8.0% by weight ofmagnesium, the balance being aluminum wherein the ratio Al/Zn is above2.9 and the coated sacrificial hardened part obtained after the methodof press hardening. These specific coatings have a native oxide layer ofa micrometric thickness. Because of the thickness and the hardness ofthe native oxide layer, these coatings are very difficult to weld.

SUMMARY OF THE INVENTION

No sufficient method has been developed to weld these specific coatedhardened parts.

It is an object of the present invention to provide an easy to implementwelding method for the manufacture of hardened parts coated withspecific coatings based on aluminum or zinc recently developed. Inparticular for the production lines, an objective is to obtain a weldingrange for such specific coated hardened parts being equal or above 1 kA.

The present invention provides a welding method for the manufacture ofan assembly comprising the following steps:

-   -   A. The provision of at least two metallic substrates (3, 3′)        wherein a first metallic substrate (3) is a hardened steel part        coated with:        -   an alloyed coating (4) comprising zinc, silicon, optionally            magnesium, the balance being aluminum, directly topped by        -   A native oxide layer comprising ZnO and optionally MgO,    -   B. The application of a spot welding cycle with a spot welding        machine, comprising welding electrodes (1,1′) and a spot welding        power source (2) applying an inverter direct current, through        the at least two metallic substrates of step A), said spot        welding cycle (21, 31, 41, 51) comprising the following        sub-steps:        -   i. one pulsation (22, 32, 42, 52) having a pulsation current            (Cp) applied through said at least two metallic substrates            joined together using welding electrodes connected to the            spot welding power source and directly after,        -   ii. a welding step (23, 33, 43, 53) having a welding current            (Cw) applied through the at least two metallic substrates            and        -   wherein the current Cp is above the current Cw and wherein            the pulsation duration is shorter than the welding duration.

The present invention also provides an assembly of at least two metallicsubstrates (3, 3′) spot welded together through at least one spot weldedjoint obtainable according to the method of the present invention, saidassembly comprising:

-   -   a first metallic substrate (3) being a hardened steel part        coated with:        -   an alloyed coating (4) comprising zinc, silicon, optionally            magnesium, the balance being aluminum, directly topped by        -   A native oxide layer comprising ZnO and optionally MgO,    -   said spot welded joint comprising a nugget (5); and said spot        welded joint being such that on its top (6), at least a part of        the native oxide layer and/or alloyed coating is not present.

The present invention also provides the use of the assembly for themanufacture of an automobile vehicle.

Other characteristics and advantages of the invention will becomeapparent from the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

To illustrate the invention, various embodiments and trials ofnon-limiting examples will be described, particularly with reference tothe following Figure:

FIG. 1 illustrates an embodiment according to the present invention.

FIGS. 2, 3, 4 and 5 illustrate Examples of spot welding cycle accordingto the present invention.

DETAILED DESCRIPTION

Other characteristics and advantages of the invention will becomeapparent from the following detailed description of the invention.

The designation hardened steel part means a hot-formed or hot-stampedsteel sheet having a tensile strength up to 2500 MPa and more preferablyup to 2000MPa. For example, the tensile strength is above or equal to500 MPa, advantageously above or equal to 1200 MPa, preferably above orequal 1500 MPa.

The invention relates to a welding method for the manufacture of anassembly comprising the following steps:

-   -   A. The provision of at least two metallic substrates wherein a        first metallic substrate is a hardened steel part coated with:        -   an alloyed coating comprising zinc, silicon, optionally            magnesium, the balance being aluminum, directly topped by        -   A native oxide layer comprising ZnO and optionally MgO,    -   B. The application of a spot welding cycle with a spot welding        machine, comprising welding electrodes and a spot welding power        source applying an inverter direct current, through the at least        two metallic substrates of step A), said spot welding cycle        comprising the following sub-steps:        -   i. one pulsation having a pulsation current (Cp) applied            through said at least two metallic substrates joined            together using welding electrodes connected to the spot            welding power source and directly after,        -   ii. a welding step having a welding current (Cw) applied            through the at least two metallic substrates and        -   wherein the current Cp is above the current Cw and wherein            the pulsation duration is shorter than the welding duration.

Without willing to be bound by any theory, it seems that the weldingmethod according to the present invention performed on two metallicsubstrates comprising at least a hardened steel part coated with thespecific coating comprising zinc, silicon, optionally magnesium, thebalance being aluminum, allows for a welding range equal or above 1 kAand a decrease of coating splashing on the assembly surface. Indeed, itseems that ZnO and optionally MgO are naturally present on the surfaceof the hardened steel part due to the oxidation of the hardened steelwith air. It is believed that the pulsation having a pulsation current(Cp) higher in intensity than the welding current (Cw) breaks at least apart of the ZnO and optionally MgO oxide layer and/or alloyed coatingpresent on the coated hardened steel part opening a path to the weldingcurrent. However, if Cp is below Cw, i.e. outside the scope of thepresent invention, it is believed that the ZnO and optionally MgObarrier layer will not be broken by the pulsation due to the importantamount of ZnO and optionally MgO. Additionally, it seems that the methodaccording to the present invention comprising one pulsation is easy toimplement on an industrial scale.

As illustrated in FIG. 1, a spot welding machine 100, comprising weldingelectrodes 1, 1′ and a spot welding source 2, is used. In this Example,the electrodes permit to join two hardened steel parts 3, 3′ coated withthe coating according to the invention 4, 4′, and 4″, 4′″ respectivelyon top of which coatings forms native oxide layers 7, 7′, 7″ and 7′″respectively. During the welding, a nugget 5 is formed between the twohardened steel parts through diffusion. The nugget is an alloy of theresidual coatings and the steel parts. Thanks to the spot welding cycleaccording to the present invention, it is believed that at least a partof the coatings 4, 4′, 4″, 4′″ are removed in the nugget. Moreover, onthe top of the spot welded joint 6, 6′, it is believed that at least apart of the native oxide layers 7, 7′, 7″, 7′″ and/or alloyed coating isnot present. Indeed, it seems that the at least one pulsation breaks thenative oxide layer and starts the welding between the coated twohardened steel parts by melting and removing the coatings on top of thespot welded joint and in the nugget. Thus, the current can flow throughthe two hardened steel parts allowing an improvement of the welding.Finally, it is believed that no cooling is needed between the at leastone pulsation and the welding step. Indeed, if a cooling is performedbetween these steps, there is a risk to stop the formation of the nuggetbetween the two hardened steel parts because the steel parts start tosolidify. On the contrary when no cooling is performed, it seems thatthe steel parts stay in liquid form and can easily be joined together.

Preferably, in step B.i), the pulsation current (Cp) is between 0.1 and30 kA, preferably between 0.1 and 20 kA, more preferably between 8.0 and20 kA and advantageously between 8.0 and 15 kA.

Advantageously, in step B.i), the pulsation duration is from 5 to 60 ms,preferably from 4 to 30 ms.

Preferably, in step B.ii), the welding current (Cw) is between 0.1 and15 kA, advantageously between 0.1 and 7.5 and more preferably between2.0 and 7.5 kA.

Advantageously, in step B.ii), the welding duration is from 150 to 500ms and more preferably from 250 to 400 ms.

Preferably, the welding force is between 50 and 550 daN.

In a preferred embodiment, the welding force during the spot weldingcycle is between 350 daN and 550 daN.

In another preferred embodiment, the welding force during the spotwelding cycle is between 50 daN and 350 daN. In this case, it seems thatthere is a better localization of current at the electrodes centersallowing a better weldability.

Preferably, the welding frequency is between 500 and 5000 Hz, morepreferably 500 and 3000 Hz and for example between 800 and 1200 Hz.

Preferably, the welding step B.ii) comprises a plurality of pulses, theat least one pulsation B.i being directly followed by the first pulse ofthe welding step. In this case, there is no cooling between thepulsation and the first pulse. The first pulse is followed by one ormore pulse(s), a break duration being present between each subsequentpulse. Preferably, the break duration is from 20 to 80 ms and preferablyfrom 30 to 60 ms.

The spot welding cycle according to the present invention can havedifferent shapes. FIG. 2 illustrates one preferred embodiment whereinthe spot welding cycle 21 has a rectangular shape comprising arectangular pulsation peak 22 and a rectangular welding peak 23. FIG. 3illustrates another preferred embodiment wherein the spot welding cycle31 has a parabolic shape comprising a parabolic pulsation peak 32 and aparabolic welding peak 33. FIG. 4 illustrates another preferredembodiment wherein the spot welding cycle 41 has a triangular shapecomprising a triangular pulsation peak 42 and a triangular welding peak43. According to other embodiments, the spot welding cycle has aparabolic and a rectangular shape comprising a parabolic pulsation peakand a rectangular welding peak or, a triangular and a rectangular shapecomprising a triangular pulsation peak and a rectangular welding peak.

FIG. 5 illustrates one preferred embodiment wherein the spot weldingcycle comprises one pulsation B.i being directly followed by a firstpulse of the welding step. In this Example, the spot welding cycle 51has a rectangular shape comprising a rectangular pulsation peak 52 andthree rectangular welding peaks 53, 53′, 53″.

The invention relates also to an assembly of at least two metallicsubstrates spot welded together through at least one spot welded jointobtainable with the method according to the present invention, saidassembly comprising:

-   -   a first metallic substrate being a hardened steel part coated        with:        -   an alloyed coating comprising zinc, silicon, optionally            magnesium, the balance being aluminum, directly topped by        -   A native oxide layer comprising ZnO and optionally MgO,    -   said spot welded joint comprising a nugget, and said spot welded        joint being such that on its top, at least a part of the native        oxide layer and/or alloyed coating is not present.

Without willing to be bound by any theory, it seems that when theassembly comprises the above specific coating on the hardened partwelded using the welding method according to the present invention, thewelding range is equal or above to 1 kA. Indeed, it seems that althoughthe thickness of the native oxide layer is higher than the coatings ofthe prior art, the welding method according to the present inventionbreaks the native oxide layer and remove at least a part of the nativeoxide layer and/or alloyed coating allowing a good weldability of theassembly.

Preferably, the alloyed coating of the hardened steel part comprisesfrom 0.1 to 40.0% by weight of zinc, more preferably between 0.1 and20.0% by weight of zinc and advantageously between 5.0 and 14% by weightof zinc and for example between 7.0 and 12.0% by weight.

Preferably, the alloyed coating of the hardened steel part comprisesfrom 0.1 to 20.0% by weight of silicon, more preferably from 0.1 to15.0% by weight of silicon and advantageously from 0.1 to 6.0% by weightof silicon and for example between 2.0 and 6.0% by weight of silicon.

Preferably, the alloyed coating of the hardened steel part comprisesfrom 0.1 to 20.0% by weight of magnesium, from 0.1 to 10.0%, preferablyfrom 0.1 to 4.0% by weight of magnesium.

Optionally, the coating comprises additional elements chosen from Sr,Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the content by weight ofeach additional element being inferior to 0.3% by weight and optionallyresiduals elements from feeding ingots or from the passage of the steelsubstrate in the molten bath including iron. For example, the amount ofiron is up to 5% by weight.

In a preferred embodiment, the second metallic substrate is a steelsubstrate or an aluminum substrate. Preferably, the second steelsubstrate is a hardened steel part according to the present invention.

In another preferred embodiment, the assembly comprises a third metallicsubstrate sheet 101 (shown schematically in FIG. 1) being a steelsubstrate or an aluminum substrate. In this case, two or several spotwelded joints are present.

Finally, the invention relates to the use of the assembly according tothe present invention for the manufacture of automotive vehicle.

The invention will now be explained in trials carried out forinformation only. They are not limiting.

EXAMPLES

All Trials being Usibor® 1500 steel sheets were hot-dip coated with acoating comprising 3% by weight of silicon, 2% by weight magnesium,10-12% by weight of zinc, the balance being aluminum. The steel sheetswere then press hardened at an austenitization temperature of 900° C.for 5 minutes.

Then, for each Trial, two identical press hardened parts were weldedtogether.

The welding range was determined according to the norm SEP1220-2.Welding test started from 3 kA and increased by 0.2 kA every two spotwelds. When two consecutive splashings occured at the same currentlevel, the splash limit was found. When the splash limit was reached,the welding current was decreased with the step of 0.1 kA to have threeconsecutive welded samples at the same current level without expulsion.This current level is defined as the upper welding limit of the currentrange: Imax.

After that, the lower limit Imin was found. Imin search was done byusing the criteria of 4√t, where t is the sheet thickness. Thiscriterion defines the minimum acceptable diameter value that guaranteedthe weld quality and strength. For confirmation five consecutive weldedsamples were obtained with superior welding diameter than minimalwelding diameter.

For Trials 1 to 12, 17 and 18, the welding cycle comprises, optionallyone pulsation having a pulsation current Cp, and one welding step havinga welding current Cw defined by Imin and Imax according the normSEP1220-2. For Trials 13 to 16, the welding cycle comprises onepulsation having a pulsation current Cp and three or four welding stepshaving a welding current Cw defined by Imin and Imax according the normSEP1220-2, a temporary stop being performed between each welding step.

The frequency was of 1000 Hz. The obtained Imin, Imax and the weldingcurrent range are in the following Table 1.

Welding step Zinc Welding percentage Pulsation current Welding in theWelding Current Number of (Cw) Cp current coating force duration (Cp)welding duration Stop Imin Imax > range Trials (wt. %) (daN) Number (ms)(kA) step(s) (ms) (ms) (kA) (kA) Cw (kA) 1 12 200 0 — — 1 — — — — - 0 212 450 0 — — 1 — — — — - 0 3 12 200 1 20 4.5 1 — — — — — 0 4* 12 200 120 10 1 340 — 4.2 5.2 yes 1 5* 12 200 1 20 11 1 340 — 4.46 5.65 yes 1.196* 12 200 1 20 12 1 340 — 4.46 5.88 yes 1.42 7* 12 200 1 20 13 1 340 —4.6 6.2 yes 1.6 8* 12 200 1 20 14 1 340 — 4.87 6.66 yes 1.79 9 12 450 120  6 1 - —  -  - — 0 10 12 450 1 20  7 1 - —  -  - — 0 11* 12 450 1 2016 1 340 — 5.67 7.07 yes 1.4 12* 12 450 1 40 10 1 340 — 5.43 6.64 yes1.21 13 12 450 1 20 10 3 160 40 5.6 6.1 yes 0.5 14 12 450 1 20 10 3 16040 4.9 5.4 yes 0.5 15 12 450 1 20 10 3 160 40 4.5 4.6 yes 0.1 16 12 4501 20 10 4 160 40 — — — 0 17* 10 200 1 20 10 1 340 — 4 5.2 yes 1.2 18* 10400 1 20 10 1 340 — 4.2 5.2 yes 1 *: according to the present invention

Trials 1, 2, 3, 9, 10 and 16 were not weldable, i.e. the criterions ofImin and Imax defined in norm SEP1220-2 were not achieved. Trialsaccording to the present invention have a welding range equal or above 1kA.

Example 2: Heterogeneous Welding Test

Usibor® 1500 steel sheets were hot-dip coated with a coating comprising3% by weight of silicon, 2% by weight magnesium, 12% by weight of zinc,the balance being aluminum. The steel sheets were then press hardened atan austenitization temperature between 900° C. for 5 minutes. They werewelded with DP600 steel grade (C: 0.14 wt.%, Mn: 2.1 wt % and Si: 0.4wt.%) coated with a zinc coating. The welding range was determined as inExample 1. The frequency was of 1000 Hz. The obtained Imin, Imax and thewelding current range are in the following Table 2.

Welding step Welding Welding Pulsation Welding Welding current Cpcurrent force Duration Stop Current duration Imin Imax > range Trials(daN) Number (ms) (ms) (kA) (ms) (kA) (kA) Cw (kA) 19* 200 1 20 — 10 3404.8 6.6 Yes 1.8 20* 300 1 20 — 10 340 5.2 6.4 Yes 1.2 *: according tothe present invention

Trials according to the present invention have a welding range equal orabove 1kA.

Example 3: Electrode Life Test

Electrode life is defined as the last weld number of a test strip beforereaching more than two welds out of eight below a minimum weld diameterdefined. The minimum weld diameter was of 4.7 mm.

Two coated hardened steel parts prepared as Trial 4 were welded togetherwith the welding method according to the present invention comprisingone pulsation and the welding step. The pulsation current was of 10 kAduring 10 ms. The welding current was Imax determined for Trial 4 inExample 1. A plurality of spot welds was performed with the electrodeson the two coated hardened parts and the weld diameter was measured foreach spot weld. Results are in the following Table 3.

Number Weld of spot diameter Trial welds (mm) 4*  10 5.2 100 5.2 200 5.2300 5.2 400 5.2 500 5.3 600 5.4 *: according to the present invention

The weld diameter was always above the minimum weld diameter with Trial4 according to the present invention.

What is claimed is: 1-22. (canceled)
 23. A welding method formanufacture of an assembly, the method comprising the following steps:A. providing at least two metallic substrates including a first metallicsubstrate being a hardened steel part coated with: an alloyed coatingincluding zinc and silicon, and optionally magnesium, a balance beingaluminum, the alloyed coating being directly topped by a native oxidelayer including ZnO and optionally MgO; B. applying a spot weldingcycle, with a spot welding machine having welding electrodes and a spotwelding power source applying an inverter direct current, through the atleast two metallic substrates of step A), the spot welding cycleincluding the following sub-steps: i. applying one pulsation with apulsation current through the at least two metallic substrates joinedtogether using welding electrodes connected to the spot welding powersource, the one pulsation having a pulsation duration, and directlyafter, ii. welding with a welding current applied through the at leasttwo metallic substrates for a welding duration; wherein the pulsationcurrent is higher than the welding current and wherein the pulsationduration is shorter than the welding duration.
 24. The welding method asrecited in claim 23 wherein in step B.i), the pulsation current isbetween 8.0 and 30.0 kA.
 25. The welding method as recited in claim 23wherein in step B.i), the pulsation duration is from 5 to 60 ms.
 26. Thewelding method as recited in claim 23 wherein in step B.ii), the weldingcurrent is between 0.1 and 15 kA.
 27. The welding method as recited inclaim 23 wherein in step B.ii), the welding duration is from 150 to 500ms.
 28. The welding method as recited in claim 23 wherein a weldingforce during the spot welding cycle is between 50 and 550 daN.
 29. Thewelding method as recited in claim 28 wherein the welding force duringthe spot welding cycle is between 350 daN and 550 daN.
 30. The weldingmethod as recited in claim 28 wherein the welding force during the spotwelding cycle is between 50 daN and 350 daN.
 31. The welding method asrecited in claim 23 wherein a welding frequency is between 500 and 5000Hz.
 32. The welding method as recited in claim 23 wherein the weldingstep B.ii) includes a plurality of pulses, the one pulsation in stepB.i) being directly followed by a first pulse of the plurality of pulsesof the welding step B.ii).
 33. The welding method as recited in claim 23wherein the spot welding cycle shape is selected from the groupconsisting of: a rectangular form including a rectangular pulsation peakand a rectangular welding peak, a parabolic form including a parabolicpulsation peak and a parabolic welding peak, a triangular form includinga triangular pulsation peak and a triangular welding peak, a parabolicand a rectangular shape including a further parabolic pulsation peak anda further rectangular welding peak, and a triangular and a rectangularshape including a further triangular pulsation peak and a yet furtherrectangular welding peak.
 34. An assembly comprising: at least twometallic substrates spot welded together through at least one spotwelded joint obtainable according to the method as recited in claim 23,the at least two metallic substrates including a first metallicsubstrate being a hardened steel part coated with an alloyed coatingincluding zinc and silicon, and optionally magnesium, a balance beingaluminum, the alloyed coating being directly topped by a native oxidelayer including ZnO and optionally MgO; the spot welded joint includinga nugget; and the spot welded joint being such that on a top at least apart of the native oxide layer or alloyed coating is not present. 35.The assembly as recited in claim 34 wherein the alloyed coating of thehardened steel part includes from 0.1 to 40.0% by weight of zinc. 36.The assembly as recited in claim 35 wherein the alloyed coating of thehardened steel part includes from 0.1 to 20.0% by weight of zinc. 37.The assembly as recited in claim 34 wherein the alloyed coating of thehardened steel part includes from 0.1 to 20.0% by weight of silicon. 38.The assembly as recited in claim 37 wherein the alloyed coating of thehardened steel part includes from 0.1 to 15.0% by weight of silicon. 39.The assembly as recited in claim 34 wherein the alloyed coating of thehardened steel part includes from 0.1 to 20.0% by weight of magnesium.40. The assembly as recited in claim 39 wherein the alloyed coating ofthe hardened steel part includes from 0.1 to 10.0% by weight ofmagnesium.
 41. The assembly as recited in claim 34 wherein a secondmetallic substrate of the at least two metallic substrates is a steelsubstrate or an aluminum substrate.
 42. The assembly as recited in claim34 wherein the second steel substrate is a further hardened steel partcoated with a further alloyed coating including zinc and silicon, andoptionally magnesium, a balance being aluminum, the further alloyedcoating being directly topped by a further native oxide layer includingZnO and optionally MgO.
 43. The assembly as recited in claim 34 furthercomprising a third metallic substrate sheet being a steel substrate oran aluminum substrate.
 44. An automotive vehicle comprising the assemblyas recited in claim
 34. 45. A method for manufacturing an automotivevehicle comprising performing the welding method as recited in claim 23.